The present invention relates to magnetic tape transport apparatus, and more particularly, to dynamic skew correction for multichannel analog recording for securing accurate phase and time relationships between signals recorded on a plurality of parallel tracks of a single recording and reproducing medium such as magnetic tape.
The recordation of information using systems having a plurality of parallel recording channels requires synchronization, alignment of phase and time relationships between the plurality of signals simultaneously recorded on the plurality of respective recording channels. This alignment is necessary to insure accurate recovery of the recorded data which would otherwise be prevented from being accurately recorded by typically occuring tape skew, jitter, wow, or flutter.
Retrieval or recovery of the data recorded on each of the channels must be accomplished substantially concurrently. Ideally, simultaneous information should be disposed on the recording tape in a line perpendicular to the longitudinal length of the tape and upon playback or reading of the tape, such information should be read back simultaneously. However, this requirement is frustrated by any physically misalignment, skew, or jitter of the tape in respect to the read-write heads of the recording systems. For example, in an extreme case, tape skew can result in information from two or more concurrently recorded groups being read serially instead of paralled as recorded. Accurate recovery of parallel recorded information requires some means of correcting or compensating for an misalignment such as for tape skew.
The phenomena known as skew may be defined as the difference in time measured between several read-back portions of differing channels. Skew is a product of two factors which result when the tape information is not recorded perpendicular to the edge of the recording tape. The first of these factors is static skew, determined by the physical alignment of the recording heads and the tape guides and also by the alignment of the original track recording heads within the head assembly. The other factor is dynamic skew, the result of the wandering and squirming of the tape as it passes across the recording head. Other factors such as the asymmetry and pattern sensitivity of the recording head also contribute to the overall skew conditions.
Permanent misalignment of heads between different recording and reproducing apparatus may be compensated for by fixed adjustments and therefore are not too serious. However, transient misalignment, due to flutter and dynamic skew of the tape, the first being due to vibration of the tape as it passes across the tape are more serious. Additionally, tape skew may be due to the sides of the tape not being cut absolutely parallel or to permanemt misalignment of the tape feed mechanism. Both effects result in the heads reproducing the individual tracks engaging the heads in echelon instead of strictly in line normal to the alignment of the tape with a consequent relative time shift displacement between the reproduced signals. All these factors are generally lumped together and referred to as "skew" because in practice they are inseparable from a measurement point of view. The total skew is the sum of the skew created when writing information on tape and the skew created when reading the same information from tape.
Tape skew accounts for a substantially large percentage of reading errors due to equipment incompatibility, such as when tapes are read by equipment different from that on which they were recorded, thus giving rise to serious compatibility problems. The real world is an analog world and much of the information and data compiled for recording is in analog form. It is common practice in recording analog information on multichannel records to correct for skewing errors and the like by digitizing the information formation and encoding the data for each channel prior to recording in order to permit subsequent digital compensation for skewing errors and the like upon playback. Such digitizing and encoding prior to recording and digital-to-analog conversion after playback requires digital apparatus needed for skew correction and entails substantial equipment expense. Accordingly, it is desirable to provide for dynamic skew correction of multichannel analog recording which avoids a necessity for digitizing and encoding the data for analog information for each channel prior to recording.